Charged particle motion inducing apparatus

ABSTRACT

Apparatus for inducing motion of charged particles in a liquid or gel using an electric field includes a region in which the motion is to be induced, first and second electrodes for generating the electric field in the region whereby a current passes between the electrodes so as to induce the charged particle motion and so as to cause ions to be received at the second electrode. A measurement apparatus is arranged to measure the amount of charge transferred between the first and second electrodes during an induced charged particle motion operation, the measurement apparatus being able to take account of any variation in current or voltage during the induced charged particle motion operation. A control system is provided for controlling a regenerating operation for regenerating the second electrode by transferring via it an amount of charge substantially equal to the measured amount so as to cause ions to be removed from the second electrode and thereby regenerate the electrode.

This invention relates to apparatus and a method for inducing motion ofcharged particles using an electric field. It relates particularly butnot exclusively to apparatus for inducing a liquid flow using anelectric field and to a method of inducing a liquid flow using anelectric field. It relates for example to pump or mixers for use inmicrofluidics.

Electrokinetic pumps use electrokinetic phenomena to provideelectrically driven fluid flows by applying a voltage and hence anelectric field to the fluid. A particular example of electrokineticphenomena is electro-osmosis. This is a well known phenomenon and isused in many different fields. It relates to the motion of polar liquidthrough a porous structure under the influence of an applied electricfield. Most surfaces possess a negative charge due to surfaceionisation. When an ionic fluid is placed in contact with the surface, alayer of cations builds up near the surface to screen this negativecharge and maintain the charge balance. This creates an electric doublelayer (EDL). When an electric field is applied across the surface, theions in the EDL are attracted towards the oppositely charged electrode,dragging the surrounding medium with them due to viscous forces. Thiscauses the fluid to move towards the negatively charged electrode.

Therefore, electro-osmosis can be used to control the movement of fluid.This has particular benefits in the field of microfluidics. Microfluidicstructures, or microsystems, consist of a series of microchannels andreservoirs, at least one dimension of which is generally in the micro-or nano-meter range and not greater than 1-2 mm. Fluids can be directedthrough these microchannels and subjected to a variety of actions suchas mixing, screening, detection, separation, reaction etc. Suchmicrostructures are of growing importance in chemical and biotechnicalfields as they allow tests and analysis to be carried out on a verysmall scale, thus reducing the amount of sample and reagents consumed ineach operation. This means work can be carried out quickly and at lessexpense than previously, with the production of fewer waste materials.Such microsystems are often referred to as “lab-on-a-chip”, orMicro-Total-Analysis Systems (μTAS).

The use of microfluidic pumps which utilise electro-osmosis isconsidered a promising technology for many microsystem applications, asthese pumps are relatively simple to fabricate and a good performancecan be obtained for a wide range of ionic concentrations.

Examples of electro-osmositic pumps are shown in WO 2004/007348.

A problem that arises in the operation of electrokinetic apparatus isthat gas bubbles may be generated electrochemically and can block theflow path. There is a particular problem caused by the generation ofhydrogen at the negative electrode. It has been proposed to address thisproblem by using certain electrode metals, such as palladium, whichabsorb hydrogen. However, after a period of operation the electrodebecomes saturated with hydrogen, and then hydrogen gas starts to form.Also, the electrode can become damaged by holding a concentration ofhydrogen which is too high, causing the metal lattice to expandirreversibly. For a typical electro-osmotic pump, saturation and bubbleformation takes place after several hours.

Viewed from a first aspect the invention provides apparatus for inducingmotion of charged particles in a liquid or gel using an electric field,the apparatus comprising a region in which the motion is to be induced,first and second electrodes for generating the electric field in theregion whereby a current passes between the electrodes so as to inducethe charged particle motion and so as to cause ions to be received atthe second electrode, measurement means arranged to measure the amountof charge transferred between the first and second electrodes during aninduced charged particle motion operation, the measurement means beingable to take account of any variation in current or voltage during theinduced charged particle motion operation, and control means arranged tocontrol a regenerating operation to regenerate the second electrode bytransferring via it an amount of charge substantially equal to themeasured amount so as to cause ions to be removed from the secondelectrode and thereby regenerate the electrode.

The first aspect of the invention also provides a method of inducingmotion of charged particles in a liquid or gel using an electric field,comprising applying a voltage to first and second electrodes forgenerating the electric field whereby a current passes between theelectrodes so as to induce the charged particle motion and so as tocause ions to be received at the second electrode, measuring the amountof charge transferred between the first and second electrodes during aninduced charged particle motion operation, whilst taking account of anyvariation in current or voltage during the induced charged particlemotion operation, and controlling a regenerating operation whichregenerates the second electrode by transferring via the secondelectrode an amount of charge substantially equal to the measured amountso as to cause ions to be removed from the second electrode and therebyregenerate the electrode.

Such a system can avoid or minimise the formation of gas bubbles at thesecond electrode. By regenerating the second electrode it is possible toavoid it becoming saturated with material carried there as ions, forexample hydrogen, and then releasing the material as gas bubbles. It isalso possible to minimise or avoid damage to the second electrode by itholding an excess of the material. In one example, the first electrodeis the positive electrode and the second electrode is the negativeelectrode.

For many liquids or gels the main electrochemical electrode reactionwill be the generation of hydrogen at the negative electrode. Theinvention can thus avoid the formation of hydrogen bubbles at thenegative electrode, and also of other gases at the positive electrodesuch as oxygen in the case of aqueous solutions and carbon dioxide inthe case of alcohol solutions.

The second electrode is preferably made of a substance capable ofabsorbing the material arriving at the electrode as ions. For example apalladium electrode will absorb hydrogen which arrives in the form ofhydrogen ions.

A microfluidic pumping scheme has been proposed in a PhD thesis byAnders Brask of the Technical University of Denmark dated 31 Aug. 2005.This involved the use of an electro-osmotic pump having a negativeelectrode made of palladium and in which the applied voltage wasperiodically reversed in order to regenerate the electrode. Liquid flowin the same direction was maintained by an arrangement of check valves,effectively rectifying the flow. The voltage was kept constant and equalin the forward and reverse phases and each phase was of equal duration.

The amount of charge transferred between the electrodes was not measuredin this system. This amount will depend on the current. The presentinventor has recognised that the current will vary over time even for aconstant voltage and constant liquid composition, because of a timedependent change in the electrodes, e.g. electrode degradation. Thefluid composition may also vary with time, depending on the use to whichthe pump is being put. It may also be desirable to vary the suppliedvoltage with time, to change the pumping pressure and flow, again inaccordance with the required use of the pump.

The present invention in its first aspect does not rely on steadyconditions during the induced charged particle motion operation e.g.liquid pumping or mixing operation. By measuring the amount of chargetransferred between the electrodes during an induced charged particlemotion operation, regeneration of an electrode can be controlled in aprecise and reliable manner.

The term “region” as used in this specification is intended to mean aregion where the electric field is generated by the electrodes, i.e. theregion generally between the first and second electrodes.

In certain preferred embodiments, the apparatus is arranged to beadjustable during the induced charged particle motion operation byvarying the voltage applied to generate the electric field. Thus it willbe possible to operate the apparatus at varying pressures and movementrates. This is useful for an e.g. microfluidic pump, whether it is to beautomatically operated as part of a lab-on-a-chip or microfuel cell, orused as a manually operated laboratory pump. Even if there is a varyingfield strength leading to a varying electric current, the measurementmeans takes account of this and measures the amount of chargetransferred, enabling electrode regeneration with the same amount ofcharge transfer.

The measurement of the charge transfer can for example be done bystandard techniques for integrating the current over a period of time.The apparatus will therefore be arranged to measure the current over aperiod of time so as to determine the amount of charge transferred. Themeasurement means can be arranged to measure automatically the amount ofcharge transferred between the first and second electrodes during aninduced charged particle motion operation. The apparatus, for examplethe control means, may store that amount in a memory. The stored amountmay be for use in a later regeneration operation, or the apparatus maytransfer charge away from the second electrode at the same rate at whichcharge arrives at that electrode, taking account of any variations.

In certain forms of the invention, the control means is arranged tocontrol the regenerating operation by effecting a current reversalbetween the first and second electrodes. The system is able tocompensate for any variation of voltage over time which is necessary forexample to adjust the induced liquid movement rate, and to compensatefor any change of the current/voltage relationship over time, forexample due to a change of fluid or fluid properties or a change of theapparatus properties such as degradation of the electrodes or othermaterials.

The induced charged particle motion may be used to induce liquidmovement. In the case of directional induced liquid flow systems,preferably a valve arrangement is provided so that liquid flow duringthe regenerating operation is in the same direction as liquid flow inthe induced liquid flow operation. However, this is not essential, sinceit may be possible for the induced liquid flow operation to take placeover an extended period, for example several hours, and then for theregenerating operation to take place at a convenient time, for exampleovernight. A simple check valve may be used to prevent reverse flowduring the regenerating operation. Alternatively the reverse voltage maybe selected to be below a minimum value which will cause flow to begenerated. In the case of electro-osmotic pumps, for example, there isusually a minimum voltage which will generate flow.

Where current reversal is to be effected, the control means may bearranged to control the regenerating operation such that theregenerating operation takes place over a longer period than the inducedliquid movement operation. Thus a lower average voltage, and current,may be used over a longer period during the regenerating operation.

The control means may be arranged automatically to switch the apparatusfrom the induced charged particle motion operation to the regeneratingoperation. This can avoid the system getting to a stage where gasbubbles are generated. It may be desirable to incorporate an indicator,which may be audible and/or visual, for indicating the status of thesystem, for example the amount of induced liquid movement capacityremaining available prior to a requirement for the regeneratingoperation to take place. Thus, for example, an alarm may be generated acertain time before regeneration will be required.

In certain preferred embodiments, the apparatus comprises a thirdelectrode and the control means is arranged to control the regeneratingoperation by passing a current between the second electrode and a thirdelectrode. In such a system, there is no need to reverse the currentbetween the first and second electrodes in order to regenerate thesecond electrode. Thus, in the case of systems having directional liquidflow, the apparatus can be operated without a check valve system forrectifying the liquid flow during current reversal.

The method may comprise using measured charge data, which relates to themeasured amount of charge transferred between the first and secondelectrodes during the induced charged particle motion operation, tocontrol the regenerating operation:

by effecting a current reversal between the first and second electrodes;or

by passing a current between the second electrode and a third electrode.

The use of a third electrode is of independent patentable significance.

Viewed from a second aspect the invention provides apparatus forinducing motion of charged particles in a liquid or gel using anelectric field, the apparatus comprising a region in which the motion isto be induced, first and second electrodes arranged to generate anelectric field in the region whereby a current passes between theelectrodes so as to induce the charged particle motion and so as tocause ions to be received at the second electrode, and a third electrodearranged to regenerate the second electrode by passing a current betweenthe second electrode and the third electrode so as to cause ions to beremoved from the second electrode.

The second aspect of the invention also provides a method of inducingmotion of charged particles in a liquid or gel using an electric field,comprising applying a voltage to first and second electrodes to generatethe electric field whereby a current passes between the electrodes so asto induce the charged particle motion and so as to cause ions to bereceived at the second electrode, and regenerating the second electrodeby passing a current between the second electrode and a third electrodeso as to cause ions to be removed from the second electrode.

As mentioned above the use of a third electrode has the advantage ofavoiding a need for current reversal to effect electrode regeneration.There is an additional benefit. In a brand new system involving twoelectrodes only and current reversal for regeneration, such as a pumpwith e.g. palladium electrodes having no absorbed hydrogen, electrolysisof the fluid takes place as a current is passed. Then, in the firstreversal of voltage and hence current, the current is usually (dependingon the fluid) mostly due to the transport of hydrogen ions from oneelectrode to the other. However, electrolysis continues to occur,creating more and more hydrogen. So in the two electrode setup, it ispossible to achieve greater apparatus and electrode longevity by usingthe voltage reversal process, and this increase in longevity can beincreased with an increased absorbance capacity, i.e. volume, of theelectrodes. However, the electrodes eventually become saturated withhydrogen after a given number of cycles, after which they are no longerbubble-free, or the electrodes disintegrate due to an excess of absorbedhydrogen. Thus, it is only possible to buy time by using the currentreversal approach to electrode regeneration and the apparatus and/or theelectrodes ultimately have a limited lifetime.

In certain embodiments of the first aspect of the invention, or in theapparatus of the second aspect of the invention, a third electrode isused. Any material absorbed by the second electrode as a result of thecurrent passed for inducing charged particle motion may be effectivelyremoved and passed to the third electrode. Material passed from thesecond electrode to the third electrode may be released at the thirdelectrode and allowed to exit the apparatus. The material may forexample be hydrogen. In the case of hydrogen, gas may be released at thethird electrode and allowed to exit the system. Thus hydrogenaccumulation in the first and second electrodes over a period of timecan be avoided.

In a system having a third electrode, it would be possible to carry outan induced charged particle motion operation and then subsequently aregeneration operation, in order to regenerate the second electrode.However, in preferred embodiments the apparatus is arranged such thatthe regenerating operation takes place at the same time as the inducedcharged particle motion operation. In such arrangements, the apparatuscan be operated without interruption for a regeneration phase. Thisavoids any inconvenience caused by interruptions and provides acontinual operation capability. If regeneration is carried out at thesame time as the induced charged particle motion operation, anaccumulation of absorbed material into the second electrode can beavoided. This reduces the risk of damage to the second electrode. Thesystem may therefore have a much longer bubble free lifetime, and theelectrodes can last longer.

The third electrode may be made of an inert material without anappreciable hydrogen absorption capability, for example platinum. Thushydrogen may form bubbles at this electrode, and the hydrogen may bevented from the apparatus. The first electrode may be made of an inertmaterial without an appreciable hydrogen absorption capability, forexample platinum. In a system having a third electrode, it is desirableto use a stable and inert material as the first electrode, since it isnot required to store material such as hydrogen.

The apparatus of the second aspect of the invention may comprisemeasurement means arranged to measure the amount of charge transferredbetween the first and second electrodes during an induced chargedparticle motion operation, the measurement means being able to takeaccount of any variation in current or voltage during the inducedcharged particle motion operation. It may comprise control means forcontrolling a regenerating operation for regenerating the secondelectrode. The control means may be arranged to control the regenerationoperation by transferring via the second electrode an amount of chargesubstantially equal to the measured amount so as to regenerate thesecond electrode.

Preferably, in those embodiments where measurement means and controlmeans are provided, the measurement means is arranged to measure thecurrent passing between the first and second electrodes, and the controlmeans is arranged to cause an equal current to pass between the secondand third electrodes at any given time. This arrangement can ensure thatregeneration occurs in real time at the rate required to maintain thesecond electrode in equilibrium.

In general, the potential of the second electrode will be intermediatethat of the first electrode and the third electrode. For example, thesecond electrode may be negative relative to the first electrode andpositive relative to the third electrode.

The difference in potential between the first and second electrodes isselected to obtain the desired performance of the apparatus, while thedifference in potential between the second and third electrodes ispreferably adjusted so as to maintain equal the amount of materialabsorbed by the second electrode as a result of induced charged particlemotion and the amount of material removed from the second electrode as aresult of regeneration, i.e. to maintain the second electrode inequilibrium. This will generally mean that the electric current betweenthe first and second electrodes and the electric current between thesecond and third electrodes are equal.

The region in which the charged particle motion is to be induced may bedefined in or contained in a passage. In certain embodiments, theinduced charged particle motion is a directional flow along the passage,and so the apparatus may operate as a pump. In other embodiments, theinduced charged particle motion takes place within the passage toachieve mixing, and so is not a directed flow along the passage. Thesecomments about the motion in the passage are applicable to the firstaspect of the invention and also the second aspect.

The third electrode is preferably contained in a chamber separate fromthe passage. The chamber may for example contain a buffered aqueoussolution. The chamber may be vented to allow escape of gas therefrom.For example it may be provided with a hydrophobic porous membrane, suchas Gore-Tex™. The vent can be used to vent hydrogen from the chamber,for example. The vent may be to atmosphere.

The second electrode may have a portion exposed to the liquid in thepassage (“a passage portion”) and a portion exposed to the inside of thechamber (“a chamber portion”). Material absorbed onto the secondelectrode during the induced charged particle motion operation can thenbe transported by diffusion to the electrode chamber portion. Forexample, hydrogen deposited at a second electrode made of palladium canbe transported in the electrode by diffusion.

The chamber is preferably provided adjacent to the passage. It maysimply be positioned to one side of the passage. In some embodiments,the chamber could have an annular form, fully or partly extending roundthe passage. The second electrode may then extend circumferentially orpart-circumferentially around the passage. For example, in a passagehaving a circular cross section, the second electrode may be a circulardisk with a greater diameter than that of the passage, so as to extendradially into the chamber. Such an arrangement provides ampleopportunity for the diffusion process in the second electrode, carryingabsorbed material out of the passage and into the chamber. Of course,the passage and/or the electrode can have other geometries, for examplerectangular or square in cross-section.

The second electrode and/or the first electrode may extend into thepassage. This applies both to those embodiments in which regenerating iseffected by current reversal between the first and second electrodes,and those embodiments in which a third electrode is provided forregeneration of the second electrode. Such an arrangement can provide agood current distribution across the passage cross-section, which may bedesirable for certain types of apparatus. For example, the firstelectrode and/or the second electrode may be perforated to allow liquidflow therethrough, whilst extending laterally of the passage, preferablyacross the entire passage, to provide an even current distribution.

In embodiments where the second electrode extends across or partiallyacross the liquid passage, material, absorbed into the electrode duringthe induced charged particle motion operation has to be transportedlaterally towards the third electrode, e.g. towards the chamber. Thismay happen by a diffiision process. If however the second electrodeforms a part of a wall defining the liquid passage, then the transportdistance (normally by diffusion) can be reduced. For example theelectrode may have a small thickness where it separates the passage andthe chamber. It may be in the form of a plate, sheet or foil. It ispreferably impervious to liquids in normal use.

In those embodiments in which the second electrode forms a part of awall defining the liquid passage, the entire second electrode may formpart of the passage wall, or alternatively part of the second electrodecould extend into the passage and part of it could form the passagewall. If the passage is generally straight, extending longitudinally,then the second electrode (or a part thereof) forming part of thepassage wall may also extend longitudinally. In an alternativearrangement the passage may be provided with a change of direction, suchas a bend, allowing the second electrode (or a part thereof) to bepositioned on the outside of the bend and hence have a relationship withthe first electrode providing a desired electric field geometry. Forexample, the passage may have a right-angled bend downstream of thefirst electrode, changing from a longitudinal direction to a lateraldirection. The second electrode may then extend in a plane generallyparallel to a plane in which the first electrode extends, both planesbeing perpendicular to the longitudinal direction. Whilst e.g. a liquidflow and pressure for inducing that flow can be generated by the voltagedifference between the first and second electrodes, once the flowreaches the bend it will be diverted from the longitudinal to thelateral direction. This arrangement can provide a short diffusion pathin the second electrode, whilst at the same time providing a goodelectric field geometry.

Another way of enhancing the electric field geometry whilst stillallowing the second electrode to provide a short diffusion path (e.g. byforming a part of the passage wall) is to provide an intermediateelectrode in the passage, between the first electrode and the secondelectrode. The intermediate electrode may for example be perforated toallow flow (of e.g. liquid) therethrough, whilst extending laterally ofthe passage, preferably across the entire passage, to provide an evencurrent distribution. Material e.g. hydrogen absorbed into theintermediate electrode during the induced charged particle motionoperation from the first electrode to the intermediate electrode may bestripped from the intermediate electrode and move as ions towards thesecond electrode, where it is absorbed and then in turn stripped by atransfer of ions to the third electrode.

The third electrode need not necessarily be provided in a separatechamber but may be provided in the passage. The amount of gas e.g.hydrogen generated at the second electrode is dependent on the currentand the volume of liquid available in which it may dissolve i.e. theliquid flow. There may therefore exist a current to flow ratio—a“critical ratio”—below which the gas may dissolve and so there is nocreation of bubbles. Thus, below the critical ratio, the gas maydissolve in the liquid and be carried away in the liquid flow. Thus, forcertain types of apparatus, at low current conditions below the criticalratio, bubbles are not created.

Therefore in low current conditions it would be possible to induce e.g.liquid movement using only the first electrode and the third electrodeprovided in the passage. No gas bubbles would be generated. At highercurrent conditions, above the critical ratio, it would be possible touse the first electrode and the second electrode during an inducedliquid movement operation. Then the regeneration operation would becarried out by applying a potential difference between the second andthird electrodes when inducing liquid movement at low currentconditions, stripping material e.g. hydrogen which had been absorbedinto the second electrode when inducing liquid movement at high currentconditions and passing that material to the third electrode from whereit can be dissolved and carried away by the flow.

The apparatus preferably comprises a power supply for supplying therequired potential difference to the electrodes. The power will normallybe supplied to provide a direct current. The power supply is preferablypart of the control means.

The charged particles may be ions, polarised molecules, other polarisedparticles such as cells, or particles with ions attached to them.

In certain embodiments the apparatus of the first and second aspects ofthe invention may be intended to generate liquid movement. When motionof the charged particles is induced, this may give rise to liquidmovement, generally as a result of viscous effects. Such liquid movementmay be used for mixing in the region between the electrodes, or adirectional flow in the manner of a pump. Thus certain preferredapparatus is arranged to operate as a pump or mixer in which liquidmovement is generated. Examples are EO pumps or EO micro mixers. Anotherexample is electrochromatography, where liquid flow is induced. Howeverthe principles involved for providing bubble free electrodes areapplicable to other systems not necessarily involving liquid movementbut where it is desired to induce motion of charged particles using anelectric field.

The invention in its various aspects is applicable to electrokineticprocesses, electrochemical processes, microfluidic and nanofluidicdevices, or laboratory devices for analysis or synthesis.

The invention in its various aspects is applicable for example toelectrophoresis or dielectrophoresis, where the first and secondelectrodes would be used to set molecules or charged particles in motionin a gel, and where separation is achieved by the different speeds ofeach species.

The invention in its various aspects is applicable to systems where anelectric field and/or current needs to be applied to an ionicallyconductive system.

The embodiments described herein are intended to provide bubble-freeelectrode systems. These have great advantages for microfluidic devices,including lab-on-chip, micro-total-analysis-systems, micro fuel cellsetc., because even small bubbles can block the flow path and disrupt theoperation of such devices.

The apparatus is applicable to all kinds of electrokinetic micro pumps,including electro-osmotic micro pumps.

Furthermore, the apparatus is usable for all kinds of microfluidicdevices and processes requiring electrodes, including electrophoresis,dipolophoresis, chromatographic techniques and dielectrophoresis. Theelectrodes can be used in all kinds of devices where bubble formationcan be a problem. It is not limited to microfluidic devices, but couldbe used in smaller (nanofluidic) and larger devices.

Certain preferred embodiments of the invention will now be described byway of example and with reference to the accompanying drawings, inwhich:

FIGS. 1 to 8 are respective schematic views of eight differentembodiments of charged particle motion inducing apparatus, in each casebeing a liquid flow inducing apparatus.

FIG. 1 shows flow inducing apparatus 10 comprising a liquid flow passage4, an inlet electrode 1 extending across the passage, and an outletelectrode 2 also extending across the passage and located at a positiondownstream of the electrode 1. Arrow 6 indicates the direction of flow.Each of electrodes 1 and 2 is perforated so as to allow flow to passthrough the electrodes. By providing perforated electrodes which extendacross the passage cross section a homogenous voltage distribution, andhence a homogenous current distribution, is provided across the passagesection. This is important for some pump designs, but is not arequirement for others.

An electro-osmotic pump 5 is provided in the flow passage. This may beany of the pump types shown in WO 2004/007348, for example. In thisembodiment, and those that are described below, the concept of providingbubble-free electrodes is described using electro-osmotic (EO) pumps.These involve the use of a porous structure with a negative surfacecharge, giving rise to a layer of positive ions building up near thesurface to screen the negative charge and maintain the charge balance.This creates an electric double layer. When an electric field isapplied, the positive ions are attracted towards the negatively chargedelectrode, and as they move they drag the surrounding liquid medium dueto viscous forces. The liquid therefore is caused to move towards thenegatively charged electrode. It will be appreciated that in alternativedesigns of EO pump, the surface charge may be positive, creating anelectric double layer with negative ions outmost and available to betransported by the electric field. With such pumps, all potentials wouldbe opposite to those described, in order to achieve flow in thedirections shown.

A voltage source V is provided between the electrodes 1 and 2, in orderto make electrode 1 positive and electrode 2 negative. An ammeter 7 isprovided in the circuit between the electrodes 1 and 2 in order themeasure the current which flows. A control system 20 is provided toreceive current data from the ammeter 7 and to provide a potentialdifference V between the electrodes 1 and 2.

The operation of the flow inducing apparatus 10 will now be described. Avoltage is applied such that electrode 1 is positive and electrode 2 isnegative, giving rise to the EO pumping effect described above. The flowof liquid in the passage 4 takes place in the direction of arrow 6. Thevoltage V is adjusted, either manually or in accordance with a programin the control system 20, in order to obtain the desired pressure andflow rate. Simultaneously the current is logged electronically byammeter 7. During this pumping process, some electrolysis of the liquidwill take place, with the generation of H⁺ ions at the positiveelectrode 1, which move to the negative electrode 2 as current carriers,driving the EO pump in the passage.

The electrode 2 is made of a material such as palladium which is capableof absorbing hydrogen. Therefore the H⁺ ions which arrive at electrode 2combine with electrons to form hydrogen atoms which are then stored inelectrode 2. However, the electrode is only capable of storing a certainamount of hydrogen, after which hydrogen gas would start to form. Inaddition, if the electrode carries too much hydrogen, its metal latticecan expand irreversibly and so be damaged.

In order to regenerate electrode 2, the voltage V applied by controlsystem 20 is reversed so that electrode 2 becomes positive and electrode1 becomes negative. Hydrogen ions form at electrode 2 and aretransported by the electric field towards electrode 1. In order to avoidback flow of liquid, the apparatus is either equipped with a check valve(not shown in the Figure), or the reverse voltage is set below a minimumvalue for flow to be generated for the EO pump 5 in use. As in thepumping operation, the current is monitored by ammeter 7 during theregeneration operation. The regeneration phase will last until the sameamount of charge (and hence of hydrogen) has been transported towardselectrode 1 as was transported in the opposite direction during thepreceding pumping phase. This is achieved by reversing the potentialsuntil the product of current and time for the regeneration phase equalsthe corresponding amount for the pumping phase. In other words, theintegrated current over the two periods is equal in absolute value andof opposite sign. In the regeneration phase, most of the current iscarried by the H⁺ ions generated from the hydrogen atoms stored inelectrode 2.

If the apparatus were to be operated in the pumping mode until theelectrode 2 approaches saturation by hydrogen, this could causeirreversible damage to the electrode. Therefore, the control system 20is arranged to stop pumping at a time before a predetermined quantity ofhydrogen has been absorbed. It may be arranged automatically to go intoregeneration mode at this point, or it may give an audible and/orvisible signal to a user. The control system may be set up to give anadvance warning that regeneration will be required.

The use of a regeneration phase following a pumping phase can thus avoidgeneration of hydrogen bubbles in the apparatus, which could otherwiseaccumulate and block the flow path. The regeneration scheme has theadditional advantage that for many liquids the main electrochemicalelectrode reaction will be the stripping and absorption of hydrogen,instead of the decomposition of the liquid. Hence, the system will notonly avoid formation of hydrogen bubbles at the negative electrode, butalso of other gases at the positive electrode, such as oxygen foraqueous solutions and carbon dioxide for alcohol solutions. However, asliquid breakdown to generate hydrogen will still take place at a lowrate, the electrodes will eventually be saturated with hydrogen, atwhich time the apparatus or the electrodes will normally have to bereplaced.

An example of the operation of the apparatus of FIG. 1 will now bedescribed. The electrodes 1 and 2 were made of perforated palladium foileach with a thickness of 25 micrometers. A typical performance at a 5 mWpower consumption and a 10 volt output from control system 20, providinga direct current, was a flow rate of 10 microlitres per minute, at apressure of 1 psi. The time before hydrogen bubbles formed at electrode2, evidencing hydrogen saturation of the electrode, was between 5 and 10hours. By using the current reversal scheme, pumping in the forwarddirection for a certain number of hours and then reversing the potentialin the regenerating phase at 5 volts for the necessary amount of time,the total operation of the apparatus was extended to four weeks duringwhich the total time of forward pumping was approximately 100 hours.

The apparatus measured the current at all times. Therefore, anyvariations in current were measured and hence taken account of to ensurethat during the regeneration phase the same amount of charge istransferred between the electrodes in the opposite direction as thattransferred during the pumping phase. It has been found that even if aconstant voltage is applied the current may vary due to changes in theliquid properties or due to long term electrode degradation. Theapparatus of FIG. 1 is arranged to allow for such variations. Inaddition, it is sometimes desirable to operate the apparatus at varyingpressures and flow rates and the apparatus of FIG. 1 is able to takeaccount of such variations.

The apparatus may be a microfluidic pump, either automatically operatedas part of a laboratory on a chip or a microfuel cell. It may be used asa manually operated laboratory pump.

In the embodiments described below, an alternative method of avoidingbubble formation at the electrodes, and hence in a flow path, isprovided. In these embodiments, current reversal is not required and sothey can provide a more continuous pumping mode. They can also avoid theneed for a check valve system to rectify the flow during a regenerationphase. Electrode regeneration does however take place, and can normallydo so at the same time as pumping, although regeneration after pumpingis an option if desired.

Referring to FIG. 2, this is similar to FIG. 1 to the extent that itshows flow inducing apparatus 10 having a flow passage 4, a perforatedinlet electrode 1 extending across the passage, a perforated outletelectrode 2 extending across the passage and positioned downstream ofelectrode 1, and an EO pump 5 between the two electrodes. The inletelectrode 1 is made of an inert metal such as platinum, whilst theoutlet electrode 2 is made of a metal capable of absorbing hydrogen suchas palladium. An input voltage (V₁) is supplied to the electrodes 1 and2 by a control system 20. When a voltage V₁ is applied a liquid flow inthe direction of arrow 6 is generated. An ammeter 7 is provided tomeasure the current passing between the electrodes 1 and 2.

In this embodiment the outlet electrode 2 extends out of the flowpassage 4 in sealed manner and into a separate liquid filled chamber 10.The chamber 10 may contain an aqueous buffer solution. In this chamber10 a third electrode 3 is provided to form a “bubbling” electrode. Thechamber 3 is closed by a semi permeable membrane 15 which allows gas toescape in the direction of arrow 9. The membrane 15 may be a hydrophobicporous membrane such as Gore-Tex. An ammeter 8 is provided in thecircuit between electrodes 2 and 3 in order to measure the currentpassing between those electrodes. The electronic data is fed to controlsystem 20. A voltage V₂ is supplied to the circuit of electrodes 2 and 3by the control system 20.

In operation, the voltage V₁ is adjusted to obtain the desired flow ratefor the given BO pump 5, while the current is monitored electronicallyby ammeter 7. Simultaneously, the voltage V₂ is adjusted electronicallyby the control system 20 so that the current measured by ammeter 8 isalways equal to that measured by the ammeter 7. In this way, the samenumber of hydrogen atoms absorbed by the electrode 2 during pumping willbe removed from the part of the same electrode extending into theseparate liquid filled chamber 10 by means of the third electrode 3. Asthe electrode 3 is made of an inert metal, such as platinum, without anappreciable hydrogen absorption capability, hydrogen will form bubblesat this electrode, from which it can be vented. The hydrogen istransported along electrode 2 by diffusion. Electrode 2 will be at apotential which is negative relative to the potential of electrode 1 andpositive relative to the potential of electrode 3.

FIG. 2 shows the separate chamber 10 only at one side of the passage 4.However, this is of course just one example. In practice, the chamber 10could encircle the passage 4 or part of it, with the outlet electrode 2extending out of the passage 4 at every point along the part or all ofthe circumference where the chamber 10 lies adjacent to the passage 4.For example, in a microchannel with a circular cross-section theelectrode 2 may be a circular disk with a greater diameter, extendinginto the separate chamber 3. The flow passage and the electrodes canalso have other geometries, for example rectangular.

It will be noted that the chamber 10 functions as a degassing chamber.

In the apparatus of FIG. 2, contrary to the two electrode regenerationscheme of FIG. 1, the palladium electrode 2 will not be saturated withhydrogen even after long periods (as can happen in the two electrodesetup due to continuing liquid electrolysis as a competing reaction tothe hydrogen stripping and absorption). As new hydrogen is generatedthis will be removed in the separate chamber 10. However, the generationof other gases at the inlet electrode 1 may still occur. Often, this isless of a problem, for example if methanol is being pumped this has alarge capacity for dissolving carbon dioxide which would otherwise begenerated at the inlet electrode 1, so that bubble formation at thiselectrode can be avoided.

It will be appreciated that in the embodiment of FIG. 2 hydrogenabsorbed into the electrode 2 diffuses laterally of the flow passage.This would mean that a hydrogen atom absorbed into the electrode at thecentre of the passage would have to diffuse by a distance approximatelyequal to half the passage diameter in order to move to the chamber 10.This creates a relatively long diffusion path, although given that theapparatus may be extremely small in size this may not be a problem.

In the embodiment of FIG. 3, the geometrical arrangement is modified toreduce the length of the diffusion path. In most respects, thisembodiment has the same arrangement of FIG. 2 and so the descriptionwill not be repeated. The difference is that the second electrode 2 doesnot extend across the passage and is not perforated. In the embodimentof FIG. 3, the second electrode 2 forms part of the wall of the passage4 and is non-perforated. Such an arrangement greatly reduces thedistance which hydrogen has to diffuse within electrode 2 between thepassage 4 and the degassing chamber 3. The electrode may be made ofplate or foil and, as with the other embodiments, is made of a hydrogenabsorbing metal such as palladium. The advantage of this embodiment overthat of FIG. 2 is that less electrode metal needs to be used to form theelectrode 2, and electrode metals such as palladium are expensive. Inthe embodiment of FIG. 2 the thickness of electrode 2 cannot be toosmall in order to allow sufficient electrode volume for fast hydrogendiffusion. In addition, with the embodiment of FIG. 3 larger currentsmay be used because hydrogen may be stripped from the electrode 2 morequickly, avoiding any part of the electrode becoming saturated withhydrogen.

A drawback of the embodiment of FIG. 3 is that pumps requiring an evencurrent distribution across the passage cross-section may not performwell with electrodes which do not extend into the passage.

This problem is addressed by the embodiment of FIG. 4. This embodimenthas the same setup in many respects as that of FIG. 3, and thedescription of those features will not be repeated. FIG. 4 differs fromFIG. 3 in that an additional electrode is provided. This is intermediateelectrode 2′ which is provided downstream of the EO pump 5. Intermediateelectrode 2′ is perforated and extends across the passage 4. It is madeof a hydrogen absorbing material such as palladium.

The intermediate electrode 2′ is arranged in series with inlet electrode1, with the control system 20 applying a voltage V₁ which sets thepotential of intermediate electrode 2′ negative with respect to that ofthe potential of inlet electrode 1. Intermediate electrode 2′ isarranged in series with the second electrode 2, with the control system20 applying a voltage V₃ between these two electrodes such thatelectrode 2 has a negative potential relative to intermediate electrode2′. An ammeter 12 is provided to measure the current flowing between thesecond electrode 2 and the intermediate electrode 2′.

During pumping, hydrogen ions are carried to intermediate electrode 2′.Because of the potential drop between electrode 2′ and electrode 2,hydrogen is stripped from the electrode 2′ and carried towards electrode2 as hydrogen ions, where it is absorbed into the electrode. Thehydrogen is then stripped from electrode 2 as hydrogen ions and carriedto electrode 3 where it forms hydrogen which may be removed as hydrogengas.

This embodiment combines the advantage of that of FIG. 2 of an outletelectrode (in this case, electrode 2′) which extends across the passageand so provides a good current distribution, with the advantage of theembodiment of FIG. 3 of a short diffusion path across the electrode 2 inthe wall of the passage. Both electrodes 2′ and 2 must be made of ahydrogen absorbing material such as palladium. As hydrogen stripping isassociated with energy transfer, this scheme (with four electrodes) willconsume more energy that the three electrode systems of FIG. 2 or 3.

In use, the voltage V₁ is adjusted to obtain the desired performance ofthe pump 5, while the voltages V₂ and V₃ are adjusted so that allcurrents are equal.

The embodiment of FIG. 4 may be modified such that intermediateelectrode 2′ is “floating”. It would not be in an external electricalcircuit with electrode 1 or electrode 2, but would be isolated. Therewould still be a potential difference between electrodes 1 and 2 todrive the flow. Intermediate electrode 2′ would still have the effect ofproviding a good current distribution across the passage. This modifiedembodiment would thus be more simple than that of FIG. 4, whilstproviding similar advantages.

FIG. 5 shows an embodiment similar to that of FIG. 3 and the descriptionof the corresponding features will not be repeated. The difference isthat the flow passage 4 includes a right angled bend. The electrode 2forms part of the wall of the passage on the outside of the bend. Thisallows the electrode 2 to face the EO pump 5 and so provide a goodelectric field distribution for the pump. At the same time, theelectrode 2 provides a short diffusion path for material e.g. hydrogenabsorbed by the electrode. As with the embodiment of FIG. 3, in use thevoltage V₁ is adjusted to obtain the desired performance of the pump 5,while the voltage V₂ is adjusted so that the currents measured by theammeters 7 and 8 are equal.

In operation of the FIG. 5 embodiment, fluid flows longitudinally in thedirection of arrow 6, driven by the electric field and the pump. When itreaches the impervious electrode 2 it is forced to change direction andso is diverted in the lateral direction shown by arrow 6 a.

The embodiment of FIG. 6 is similar to that of FIG. 5 in many respectsand the description of the corresponding parts will not be repeated. Inthe FIG. 6 embodiment, the EO pump 5 is placed adjacent to the secondelectrode 2. The second electrode 2 has a non-perforated part 2 a and aporous part 2 b. The two parts are formed as two layers arranged face toface. The arrangement allows flow through the EO pump parallel to theelectric field created by electrodes 1 and 2, in the direction of arrow6, with the flow then blocked so as to cause the flow to be divertedperpendicular to the electric field in the direction of arrow 6 a.

The presence of the porous part 2 b of the electrode permits the lateralflow through the porous part. This makes it possible for the EO pump 5to be located directly adjacent to the electrode 2 without blocking theflow. For some types of pump, it is advantageous to be able to positionthe pump directly next to the outlet electrode.

The embodiment of FIG. 7 is similar to that of FIG. 5 in many respectsand the description of the corresponding parts will not be repeated. Inthe embodiment of FIG. 7 a single voltage supply V₄ is used, connectedwith its two poles connected to the first electrode 1 and the thirdelectrode 3. A voltmeter 11 is provided to measure the voltage betweenelectrodes 1 and 2.

In use, as the current will necessarily be the same between electrodes 1and 2 as between electrodes 2 and 3, so the hydrogen removalfunctionality will be retained when hydrogen is the main positivecurrent carrier in both the passage and the separate chamber. Thevoltage V₄ will be adjusted so as to adjust the voltage measured atvoltmeter 11 and thereby obtain the desired flow and pressure. Thecurrent in the circuit can be measured by ammeter 7, in order to ensurethat it does not become too high.

Similarly, only the inlet 1 and degassing 3 electrodes need be connectedto the voltage supply in the case of four (or more) electrodes e.g.provided along a passage. The voltage between the two electrodesdefining the voltage across the pump must always be monitored and thevoltage supply adjusted to get the desired voltage across the pump.

The embodiment of FIG. 8 differs from those of FIGS. 2-7 in that noseparate degassing chamber 3 is provided. This embodiment has a flowpassage 4 in which is provided an EO pump 5 through which liquid flowsduring pumping in the direction of arrow 6. An inlet electrode 1 isprovided upstream of the pump 5, a second electrode 2 forming a firstoutlet electrode is provided at a first location downstream of the pump5, and a third electrode 3 forming a second outlet electrode is provideddownstream of the second electrode 2. All three electrodes extend acrossthe flow passage to provide an even current distribution and areperforated to allow flow therethrough. The first and third electrodesare formed of non-hydrogen absorbing material, such as platinum, whilstthe second electrode is formed of a material capable of absorbinghydrogen, such as palladium. The control system 20 applies a voltage V₁between electrodes 1 and 2 and a voltage V₂ between electrodes 2 and 3.An ammeter 7 measures the current flowing between electrodes 1 and 2 andan ammeter 8 measures the current flowing between electrodes 2 and 3.

The amount of hydrogen generated depends on the current, and thepossibility for hydrogen to be dissolved depends on the available liquidvolume, i.e. the flow. There therefore exists a current to flow ratio (acritical ratio) below which hydrogen generated at an electrode dissolvesand so does not create bubbles.

In the embodiment of FIG. 8, if a current is flowing which is below thecritical ratio the electrodes 1 and 3 are used to generate the electricfield and operate the pump 5. Hydrogen ions arriving at the thirdelectrode 3 are converted to hydrogen which dissolves in the liquid andis carried away.

When it is desired to increase the flow such that the current will beabove the critical ratio, then electrodes 1 and 2 may be used. Hydrogenions arriving at electrode 2 are absorbed into the material of theelectrode. Then, when pumping using a current below the critical ratio,a potential difference may be applied between electrodes 2 and 3 suchthat the hydrogen absorbed into electrode 2 is stripped away as hydrogenions which are carried to electrode 3, where they generate hydrogenwhich dissolves in the liquid. Hydrogen ions will thus arrive atelectrode 3 as a result of the current flowing from electrode 1 throughthe pump and also as a result of hydrogen being stripped from electrode2. Therefore the current flowing between electrodes 1 and 2 needs to bebelow the critical ratio with a sufficient margin to allow for the extrahydrogen which will be carried to electrode 3.

An optional addition would be to provide a separate chamber into whichelectrode 2 would extend, increasing the capacity for hydrogen removal.

The advantage of the FIG. 8 embodiment is that by dissolving thehydrogen generated in the liquid it is possible to save energy and makethe hydrogen absorbing material of electrode 2 last longer. The criticalratio can be calculated when the hydrogen absorbing capacity of theliquid is known. Alternatively, a commercial bubble detector can beconnected to the control electronics, which will switch to the use ofthe hydrogen absorbing electrode to when required.

The system will typically be controlled by standard low voltageelectronics. The various power supplies (symbol V), ammeters (A) andvoltmeters represent just functionalities of electronic circuitry. Thesame circuit will typically adjust the voltage to obtain the desiredliquid flow and pressure, while automatically taking care of theelectrode regeneration.

The main features and advantages of the various preferred embodimentsare as follows.

The current integration method assures that the pump can run for longperiods without hydrogen saturation (resulting in bubble formation) forthe case of a two electrode system, and indefinitely without bubbles forthe three or more electrode system.

Although a regeneration scheme with two electrodes has been proposed inthe prior art, the method does not compensate for variation of voltageover time which is necessary to adjust the pumping rate, and does alsonot compensate for the change of the current—voltage relation over time(due to change of fluid or change of the pump properties like long termdegradation of electrodes and other materials). This strongly limits theknown method, which is based on applying equal voltage for equal time inboth directions.

The three (or more) electrode system also has the new feature thathydrogen will not accumulate slowly in the electrodes over time, hencethe system will have a much longer bubble free lifetime, and theelectrodes will also last much longer. In addition, there is no need forreversing the flow to regenerate the electrodes.

The two electrode system has the advantage that the current can mainlybe carried by hydrogen ions, which is removed from one electrode andabsorbed by the other. After a short initial hydrogen generation period,the formation of other gases than hydrogen at the negative electrode(typically O₂ for aqueous and CO₂ for alcohols) will also be suppressed.For some liquids this can be important, although in other cases thisbrings no advantage as the other gas can be dissolved in the liquid (forexample, methanol and ethanol have large capacities for absorbing CO₂,while much smaller H₂ absorbance).

Bubble free electrodes have great advantages for microfluidic devices,including lab-on-chip, micro-total-analysis-systems, micro fuel cellsetc, as even small bubbles can block the flow path and disrupt theoperation of such devices.

The pump should be ideal for all kinds of electroosmotic/electrokineticmicropumps.

Further, it should be ideal for all kinds of microfluidic devices andprocesses requiring electrodes, including electrophoresis,dipolophoresis, chromatographic techniques, dielectrophoresis.

The electrodes can be used in all kinds of devices where bubbleformation can be a problem. It is not limited to microfluidic devices,but could be used in smaller (nanofludic) and larger devices.

At least in the preferred embodiments, the regenerating operation iscontrolled so as to regenerate the second electrode by transferring viathe second electrode an amount of charge equal to the amount measuredduring the induced charged particle motion operation, so as to causeions to be removed from the second electrode and thereby regenerate thatelectrode.

1. Apparatus for inducing motion of charged particles in a liquid or gelusing an electric field, the apparatus comprising a region in which themotion is to be induced, first and second electrodes for generating theelectric field in the region whereby a current passes between theelectrodes so as to induce the charged particle motion and so as tocause ions to be received at the second electrode, measurement meansarranged to measure the amount of charge transferred between the firstand second electrodes during an induced charged particle motionoperation, the measurement means being able to take account of anyvariation in current or voltage during the induced charged particlemotion operation, and control means arranged to control a regeneratingoperation to regenerate the second electrode by transferring via it anamount of charge substantially equal to the measured amount so as tocause ions to be removed from the second electrode and therebyregenerate the electrode.
 2. Apparatus as claimed in claim 1, whereinthe apparatus is arranged to be adjustable during the induced chargedparticle motion operation by varying the voltage applied to the firstand second electrodes.
 3. Apparatus as claimed in claim 1, wherein thecontrol means is arranged to control the regenerating operation byeffecting a current reversal between the first and second electrodes. 4.Apparatus as claimed in claim 3, wherein the control means is arrangedto control the regenerating operation such that the regeneratingoperation takes place over a longer period than the induced chargedparticle motion operation.
 5. Apparatus as claimed in claim 3, whereinthe control means is arranged automatically to switch the apparatus fromthe induced charged particle motion operation to the regenerationoperation.
 6. Apparatus as claimed in claim 1, comprising a thirdelectrode, and wherein the control means is arranged to control theregenerating operation by passing a current between the second electrodeand a third electrode. 7-13. (canceled)
 14. Apparatus for inducingmotion of charged particles in a liquid or gel using an electric field,the apparatus comprising a region in which the motion is to be induced,first and second electrodes arranged to generate an electric field inthe region whereby a current passes between the electrodes so as toinduce the charged particle motion and so as to cause ions to bereceived at the second electrode, and a third electrode arranged toregenerate the second electrode by passing a current between the secondelectrode and the third electrode so as to cause ions to be removed fromthe second electrode.
 15. Apparatus as claimed in claim 14, arrangedsuch that the regenerating operation takes place at the same time as theinduced charged particle motion operation.
 16. Apparatus as claimed inclaim 14, comprising measurement means arranged to measure the amount ofcharge transferred between the first and second electrodes during aninduced charged particle motion operation, the measurement means beingable to take account of any variation in current or voltage during theinduced charged particle motion operation.
 17. Apparatus as claimed inclaim 14, comprising control means for controlling a regeneratingoperation for regenerating the second electrode.
 18. Apparatus asclaimed in claim 17, wherein the control means controls the regenerationoperation by transferring via the second electrode an amount of chargesubstantially equal to the measured amount so as to regenerate thesecond electrode. 19-32. (canceled)
 33. A method of inducing motion ofcharged particles in a liquid or gel using an electric field, comprisingapplying a voltage to first and second electrodes to generate theelectric field whereby a current passes between the electrodes so as toinduce the charged particle motion and so as to cause ions to bereceived at the second electrode, and regenerating the second electrodeby passing a current between the second electrode and a third electrodeso as to cause ions to be removed from the second electrode.
 34. Amethod as claimed in claim 33, wherein the regenerating operation takesplace at the same time as the induced charged particle motion operation.35. A method as claimed in claim 33, comprising measuring the amount ofcharge transferred between the first and second electrodes during aninduced charged particle motion operation, such measuring taking accountof any variation in current or voltage during the induced chargedparticle motion operation.
 36. A method as claimed in claim 33,comprising controlling the regenerating operation for regenerating thesecond electrode.
 37. A method as claimed in claim 36, wherein theregeneration operation is controlled by transferring via the secondelectrode an amount of charge substantially equal to the measured amountso as to regenerate the second electrode.
 38. (canceled)
 39. A method asclaimed in claim 33, comprising releasing material passed from thesecond electrode to the third electrode at the third electrode.
 40. Amethod as claimed in claim 9, wherein said material is hydrogen.
 41. Amethod as claimed in claim 33, comprising setting the potential of thesecond electrode intermediate that of the first electrode and that ofthe third electrode.
 42. A method as claimed in claim 33, wherein thethird electrode is made of an inert material without an appreciablehydrogen absorption capability. 43-47. (canceled)